1. Field of the Invention
The present invention relates to an image capturing apparatus and a control method of the image capturing apparatus, and more particularly, to an image capturing apparatus that uses an ND filter to control exposure and that has a zoom function and a control method of the image capturing apparatus.
2. Description of the Related Art
Conventionally, a lens related to zoom magnification control or a group including the lens in a lens unit is controlled back and forth to change and control the zoom magnification in an image capturing apparatus, such as a digital still camera and a digital video camera, with a zoom function. However, a change in the zoom magnification usually changes the focal length, which causes a change in the minimum f-number due to attenuation of the quantity of incident light, and the exposure is changed. For example, if the zoom magnification is controlled to the maximum (telephoto end) when the specification of the lens is “F1.8 to F2.8”, the quantity of incident light is attenuated even if the aperture diameter of the diaphragm is open, i.e. opened to the maximum extent. As a result, the exposure is reduced, and the minimum f-number increases. Under such a specification, the minimum f-number cannot be set to F1.8 even if the aperture diameter of the diaphragm is open, and the minimum f-number increases to F2.8 at the maximum as a result of the exposure change due to the change in the zoom magnification.
In zoom lenses of many image capturing apparatuses, a group of lenses arranged in front of the diaphragm (group closer to the subject than the diaphragm) is moved back and forth over the optical axis to control the zoom magnification. Therefore, if the diaphragm is controlled to the aperture diameter with an f-number greater than the minimum f-number (for example, F2.8 in the example of specification described above) in which the zoom magnification is determined at the telephoto end, the zoom can be operated while keeping the f-number constant. More specifically, the exposure can be maintained at an aperture diameter of the diaphragm corresponding to the f-number greater than the minimum f-number at the telephoto end even if the zoom magnification is changed. However, the f-number is changed by the zoom magnification at an aperture diameter greater than the aperture diameter of the diaphragm corresponding to the minimum f-number determined at the telephoto end even if the aperture diameter of the diaphragm is the same.
Meanwhile, demand for miniaturization and wider angle of the image capturing apparatus is increasing, and there can be a zoom lens that controls the zoom by controlling not only the group of lenses arranged in front of the diaphragm, but also a group of lenses arranged on the back (group closer to the image capturing apparatus than the diaphragm), back and forth over the optical axis. Unlike the control of only the group in front of the diaphragm, the focal length on the back of the diaphragm changes relative to the aperture diameter of the diaphragm when such a zoom lens is used. Therefore, there is a problem that the f-number changes in accordance with the zoom magnification throughout the entire aperture diameter of the diaphragm.
Consequently, a first method is proposed, which is a method of correcting the f-number and keeping the exposure by controlling the size of the aperture diameter of the diaphragm as for the change in the f-number relative to the change in the focal length caused by the zoom control. For example, there is a proposed method which provides a table, in which the size of the aperture diameter of the diaphragm relative to the f-number is defined for each focal length that can be resulted from the zoom control of the zoom lens, and the table is used for the correction. In the zoom control of this method, the size of the aperture diameter of the diaphragm defined in the table is calculated based on the zoom magnification and the current f-number, and then the diaphragm is controlled to the calculated aperture diameter to enable the zoom control while maintaining the exposure (for example, see Japanese Patent Laid-Open No. 8-279957).
A second method is proposed, which is a method using a gain circuit, an electronic shutter circuit, etc. In the zoom control of this method, the gain circuit performs the correction based on a gain value so as to cancel the exposure change in accordance with the change in the focal length caused by the zoom control. Similarly, the electronic shutter circuit controls the accumulation of the image sensor to compensate the exposure change (for example, see Japanese Patent Laid-Open No. 9-51473).
However, when the first method is applied to the conventional image capturing apparatus using the ND filter, the state of the area, position, density, etc., of the ND filter that covers the aperture diameter may change as shown in FIGS. 5A and 5B. Particularly, the state may change when an ND filter with gradationally changing transmittance or density is used or when the ND filter partially, not completely, covers the aperture diameter. In that case, the exposure change caused by the change in the state of the ND filter is prominent.
In the second method, the exposure change caused by the change in the focal length is corrected without controlling the size of the aperture diameter of the diaphragm, and the state of the area, position, density, etc., of the ND filter covering the aperture diameter does not change unlike the first method. Therefore, the exposure is not changed by the ND filter. However, when the gain circuit is used, electrical correction is performed by enlarging the gain value for the exposure change, and a noise is generated in the captured image. When the electronic shutter circuit is used, the shutter speed may be slowed down by the correction to the slow shutter speed, in which case the motion resolution is reduced. Therefore, it is difficult to obtain an excellent image.